In industrial finishing applications, coating particles are emitted from the spray device such as a spray gun toward an object to be coated. One type of coating material is a solid powder coating which is finely ground into powdered or particulate form for spraying onto parts in a manner similar to liquid paint. In electrostatic powder spray guns, one or more electrodes associated with the spray gun imparts an electrostatic charge to the coating particles as they are emitted toward the object to be coated. The object to be coated is maintained at an electrostatic potential different from that of the charged coating particles so that the coating particles are attracted to the article and deposited thereon with improved efficiency and coverage. The electrostatic charge maintains the powder upon the product for a sufficient time period to permit the powder to be heated so that it melts, and when subsequently cooled, the powder is firmly attached to the target substrate.
Electrostatic spray guns, particularly those designed for spraying particulate powder material, include a particle deflector mounted at the nozzle end of the spray gun. In one preferred form, the particle deflector is in the shape of a cone which is mounted axially in the flow path of the particulate powder material sprayed from a discharge opening in the nozzle of the spray gun. The particles impact the cone and are deflected radially outwardly into a conical spray pattern for deposition onto the article to be coated.
A primary goal of electrostatic spray guns is to maximize "transfer efficiency", i.e., the efficiency with which charged coating particles sprayed from the gun are deposited on an article to be coated. It has been disclosed in co-pending U.S. patent applications, Ser. No. 724,392, now abandoned entitled "Powder Spray Gun", filed Apr. 18, 1985 by Sharpless et al, and owned by the same assignee as this invention, and a related application, Ser. No. 07/072,780, filed July 13, 1987, and entitled "Particle Spray Gun", that the transfer efficiency of powder spray guns can be maximized in at least two ways. One way of increasing transfer efficiency is to form the particle deflector with a resistive sheet sandwiched between its forward and rearward ends, leaving exposed only the perimeter of the resistive sheet at the periphery of the particle deflector. The resistive sheet is electrically connected to a high voltage electrostatic cable and its perimeter functions as a multi-point electrode which is positioned by the particle deflector near the discharge opening of the nozzle of the gun directly in the path of the particle stream.
Additionally, as disclosed in patent applications Ser. Nos. 724,392 and 07/072,780, transfer efficiency is increased by providing an electrostatic shield at the forward end of the gun barrel. The electrostatic shield is disposed outwardly of the perimeter of the deflector where the corona charging points of the resistive sheet are located. In the preferred form disclosed in the above-identified applications, the electrostatic shield is formed by flaring the end of the nozzle in the region surrounding the forward end of the conical deflector, particularly the perimeter thereof, from which the corona charging points extend.
The electrostatic shield provided by the flared end of the nozzle in the prior art has the effect of preventing the charged particles emitted from the nozzle of the gun from being exposed or "seeing" grounded objects such as the handle of the gun, the gun mounting hardware, objects surrounding the gun in the spraying environment and the like. Because such grounded objects can provide an attraction to some of the charged particles, a corona current path can be set up between the charged particles at the electrode and one or more grounded objects which causes the available electric energy for charging at the deflector to be reduced by parasitic discharge. In the absence of an electrostatic shield to prevent such parasitic discharge, the reduction in available charging energy at the deflector results in a corresponding reduction in transfer efficiency.
The deflector of the type disclosed in the U.S. Ser. Nos. 724,392 and 07/072,780 applications has been employed in an electrostatic powder spray gun of the type disclosed in U.S. patent application Ser. No. 07/054,746, filed May 27, 1987, and entitled "Electrostatic Spray Gun Device and Cable Assembly" to Sharpo less et al. In the spray gun disclosed in that application, the deflector is directly connected to a high voltage electrostatic cable which is axially adjustable to vary the axial position of the deflector with respect to the powder discharge opening in the nozzle of the gun. Such adjustment of the deflector enables the width of the spray pattern produced by the electrostatic spray gun to be varied, while obtaining improved transfer efficiency from the multi-point electrode carried in the particle deflector.
One limitation of the electrostatic spray gun disclosed in the '746 application is that transfer efficiency can decrease when the width of the spray pattern is varied. For example, if the deflector is moved axially forwardly relative to the discharge opening in the nozzle in order to decrease the width of the spray pattern, the effect of the electrostatic shield provided by the flared end of the nozzle is reduced. This can result in parasitic discharge along a corona current path between the electrode and grounded objects in the environment in which the spray gun is operated and thus lessen the transfer efficiency of the spray gun.